Current LCD screens with white LED backlight technology use a variety of configurations of the LED backlights. In these devices, the color of the picture is generated by opening and closing the LCD's; the brightness of the picture is provided from the LED backlight.
In these screens, there is more than one method of controlling the brightness of these screens that a human observer perceives. For example, some devices control the current that flows through the LED's, as more current through a LED tends to make the LED brighter. The downside of this method is that altering the current through an LED skews the color spectrum that the LED produces, making the expected white color warmer or cooler.
An alternate method is to cycle the LED's on and off at a rate that is too high for the human eye to perceive, but to maintain a constant current through the LED's while they are on. This PWM algorithm controls the brightness by controlled the fraction of time that the LED's are producing light; the higher the percentage of time that the LED's are on, the brighter the display appears. For example, an LED might have a constant PWM frequency of 1000 hertz, and for each of those 1 millisecond PWM cycles, it might be on for 100 of the 1000 microseconds of that cycle; such an LED would be said to have a duty cycle of 10%. Since the current is constant during the 10% of the time that the LED is on, the color spectrum produced by the LED remains constant.
Consider FIG. 1: a serially wired string of a switch, a resistor and six LED's with forward voltages of about 3.5 volts each, wired to a 24 volt power supply. When the switch allows current to flow, the resistor and the forward voltages of the six LED's will define the current, and that current will not change as the frequency of the switching increases or decreases. In the drawing, with a “perfect” 10.0 ohm resistor and a string of six “perfect” LED's with forward voltages of 3.5 volts each—21 volts total—the amount of current would 0.3 amps. (The voltage across the resistor is 24−21=3 volts; using Ohm's law, the current is 3 volts/10 ohms, or 0.3 amps.)
However, manufacturing variations invariably result in imperfect parts. Consider the case where the resistor is 10.3 ohms rather than 10.0 ohms, while the forward voltage of the string of LED's is 22 volts rather than 21 volts. In this revised example, the current is 2 volts/10.3 ohms, or 0.194 amps, very different from the 0.3 amps “perfect case.”
A small LED backlit display, such as that of a small television, might contain two such identical strings of serially connected LED's that share the same switch such as is illustrated in FIG. 2. In this television, manufacturing variation in the production of the resistors and the LED's would lead to different sides of the television having different brightnesses and different color spectra because the current through each string is not managed—it is set passively by each string's combination of resistor and LED's.
In the case of this television, a more sophisticated method of controlling the current that passes through the LED's is needed. If, as in FIG. 3, the resistors are replaced by current sinks, the amount of current can be controlled in each of the two strings, and the brightness and color spectra of the strings made equal. (The current sinks and the switch must be synchronously controlled such that the current sinks are active only when the switch is passing current.) In this example, the PWM timings of the switch will control the perceived brightness of the LED's, while the current sinks will stabilize the color spectrum at the desired level by controlling the flow of current through each string and keeping the two currents the same.
Problems start to arise when the voltage produced by the power supply inadequately matches the sum of the voltages required by the current sinks and the strings of LED's—the supplied voltage might be too high or too low. For example, 24 volt power supplies are common in LCD televisions, but a string of 8 LED's, each with a forward voltage of about 3.5 volts, would require a higher voltage in order to operate efficiently. A string of 3 or 4 LED's, on the other hand, would require a lower voltage to operate well.
Because efficiency is a major requirement, it is desirable to introduce a switching regulator—either a boost converter that regulates the voltage upwards, or a Buck converter that regulates the voltage downwards—into the circuit. This introduction brings with it a new problem.
Consider FIG. 4, in which boost converter circuitry has been introduced to raise the voltage from the input power source to the single string of LED's. What is desired is a boosted voltage that is high enough to run to both the strings of LED's and the current sink, but low enough to maintain efficiency. Because each manufactured LED has a variable forward voltage, each manufactured circuit's optimal voltage will be different.
In this simple circuit, the Driver Logic Module controls the switching that surrounds the inductor, while the error amplifier ensures that there's enough voltage left on the string—after the LED's forward voltages take their share—to run the current sink. If the measured voltage at the point between the LED's and the current sink is too low, the feedback mechanism controlled by the error amplifier will attempt to raise the voltage produced by the controller by allowing the inductor to charge for a longer period of time before it is discharged through the LED's.
This straightforward solution works well when the LED's and the current sinks are generally on or off, when the fraction of time that it takes the boost converter to supply the optimal voltage from an OFF state is negligible or not noticeable as compared to the time that the LED strings are actually on. When the circuit of FIG. 4 starts from an off state and turns the LED's on, it can take the error amplifier quite a bit of time to find the proper timing required of the inductor switch in order to provide that optimal voltage. Because the voltage at the output of the error amplifier directly controls the peak current that runs into the converter, a starting state where the voltage has been artificially driven to an incorrect level will require the error amplifier to “hunt around” as it endeavors to bring order to the circuit.
This solution does not work well when the LED strings are pulsed ON and OFF at high PWM frequencies. As before, this high frequency PWM is desirable because it varies the brightness of the LED strings by combining “light times” and “dark times” in each PWM cycle. What is a desired is a system that supports a high PWM rate while, at the same time, regulates current such that there is no noticeable variation during the startup period in which the boost converter might experience large load transients. The higher the PWM frequency, the less acceptable these transients become.
For example, consider a system in which the desired PWM rate is not 1 hertz but rather 1,000 hertz. If, during each 1000 microsecond PWM cycle, the error amplifier takes 100 microseconds to adapt, the unwanted behavior would occur for 100,000 microseconds of each second—10% of the total time!